There are many methods for measuring the permittivity and permeability of materials. Among those are the slab method where a slab of material is inserted into a waveguide or coaxial transmission line, and its electrical properties are deduced from a measurement of the phase and magnitude of reflection and transmission through the slab. Such measurements are known as S-parameters. Another method is the microstrip method where the material is used as the substrate for a microstrip transmission line, and the material properties are deduced from the measurements of S parameters at each end of the transmission line. The microstrip method is most commonly used for characterizing the material used in electronic circuit boards. Another method is the cavity method, where the material is inserted into a resonant cavity, and its properties are deduced from measurement of the resonant frequency of the cavity. Another method is to use an impedance analyzer, such as Agilent model No. 4294A, where the electric and magnetic properties are deduced from measurements of the capacitance and inductance of a capacitor and an inductor respectively loaded with the sample material. These are a few of the most common methods used to measure electrical properties. Many more exist.
An important parameter in many applications is the loss the material presents to RF fields due to the complex part of its permittivity and/or its permeability. In many RF applications, it is desirable to use materials with very low loss. Even a very low loss can have a serious impact on the performance of RF devices realized in integrated circuits, such as power amplifiers, low-noise amplifiers, couplers, analog-to-digital converters, phase discriminators, etc. The figure of merit for material loss is known as the loss tangent, which is the ratio of the imaginary part to the real part of the permittivity or the permeability. In some applications, it is important to be able to distinguish between a material with 0.001 loss tangent and 0.002. Measurement of loss tangent to such a degree of accuracy is a formidable task. The impedance analyzer can measure low loss tangents to about an accuracy of 0.001 but it requires the fabrication of two different shaped samples in order to measure both electric and magnetic properties. The microstrip line method gives good measurement of loss if the material lends itself to being fabricated into the microstrip line. However, it is almost exclusively used for materials with no magnetic properties.
Of the methods mentioned above, the cavity method provides the most accurate measurement of low loss tangents because the loss has a major impact on the cavity resonance. At the cavity resonance, the fields in the cavity are very high, and the effects of the material loss on the measurement are magnified. The disadvantage of the cavity method is that it yields the properties at the specific frequency of the resonance. This is satisfactory if the material properties are independent of frequency, or if the resonance frequency is the frequency of interest that the material will be used for. In general, material electric properties vary with frequency, and in some case, they vary greatly. For that reason, it is desirable to be able to measure the frequency dependence of the material properties. For that reason, the cavity resonance method is often implemented with a cavity whose resonant frequency can be tuned by some means. The most common method of tuning is to make the cavity with a mechanically adjustable dimension. For example, one of the cavity walls can be made to slide in and out relative to the other walls. As it slides in, the cavity resonance is driven to higher frequency and vice versa. Another method is to attach an adjustable shunt to the cavity coupled to a phase shifter. As the phase shifter is varied, the cavity resonance is tuned over a limited frequency range.
Sometimes it is desirable to measure material properties at low frequencies where the wavelength is large, sometimes more than a meter or more. Of the methods mentioned above, the impedance analyzer offers the most convenient measurement method at low frequency less than 1 GHz. However, its ability to measure low loss tangents with great accuracy is limited. Also, the technician is required to machine the sample into samples with extremely tight dimensional accuracy for the measurement. Inserting the sample into a coaxial transmission line and measuring its S parameters to deduce its properties is a reliable method for measuring properties over a wide frequency range, but it doesn't provide great accuracy for low-loss materials.
Despite the plethora of property measurement methods, there still exists a need for a way to accurately and simultaneously measure the electric and magnetic properties of low loss tangents materials to high accuracy over a large frequency range with a single material sample in a single device. The embodiments of the present disclosure answer these and other needs.